Electric hair dryers are systems that produce heated air streams to dry hair. They have electric motor driven fans that suck in ambient air, compress and accelerate it, then channel the air stream over electric resistance heaters onto the users hair. As the same amount of air that is discharged from a dryer must be taken in, the possibility of foreign matter, especially hair, being sucked into the dryer is always present.
While dust and other particulate matter just passes through the dryer, hair has a tendency to wrap around turning spindles and objects. This problem is especially acute with hand held dryers that are used in or closely adjacent to hair. Further, hand held hair dryers use fast running motors that produce high velocity air streams. Hair entering the intake has a tendency to wind around the motor shaft and fill up the space between the rotating fan and the stationary bearing until the hair finally jams the fan.
With axial fans, where a close clearance between the tip of the fan and the shroud is maintained, hair causes an additional problem: Hair strands wind around the outside diameter of the fan, fill up the clearance gap and again jam the fan. Such a buildup of hair either about the motor shaft or in the small clearance space between the fan tip and fan shroud results in an additional torque load being placed on the motor which can greatly reduce the expected useful life of the motor. Also the motor speed is reduced causing less air to flow over the heating elements and resulting in the possible activation of the thermal shut off switch for the dryer due to excessively high air temperatures being reached in the dryer housing. This could be a great inconvenience since the dryer would be rendered useless to the user until proper servicing was made to the dryer to reset the shut off switch.
Attempts have been made to prevent hair from entering dryers by the use of screens at air inlets. They have not proven successful. Fine mesh screening or filtering requires extremely large intake areas and leads to clogging by airborn dust and dirt. Attempts to provide labyrinth guards between the rotating and stationary elements of a dryer were not successful as hair has a diameter of about 0.1 mm, and in a consumer product, which is mass produced, clearances of this order cannot be maintained. The use of contact seals, on the other hand, are wear points and require additional motor power which is not feasible in trying to keep the dryer of a compact size for portability.
It is pointed out that the prior art discloses the use of serrated or abrasive surfaces in pumping mechanisms for pumping liquids (e.g. U.S. Pat. Nos. 3,953,146 to Sowards and 3,961,758 to Morgan), as well as cutter knives in vacuum cleaners for cutting tape or yarn (.e.g. U.S. Pat. No. 3,060,483 to Black). However, there is no disclosure of hair dryers having abrasive means for cutting hairs which tend to jam the motor shaft thereby preventing rotation of the fan.